Motors having a rotor and stator assembly are used in home appliances, industrial equipment, computer disc drives and hybrid electric vehicles.
Two types of stator assemblies have been available for use in motors or generators depending on the way the wire coil is wound on the stator, i.e., one is a distributed coil stator assembly and the other is a concentrated coil stator assembly. The concentrated coil stator assembly has the advantage of utilizing a shorten coil end, thereby downsizing the motor. The shortened coil of this stator assembly reduces the copper loss generated by wire-wound-coil and forms a highly efficient motor or generator.
A number of cooling methods have been used to cool motors and generators, e.g., dissipating fins on a frame arranged around the stator rim to substantially cool the surface area, a forced air cooling system using a fan, or a liquid cooling system that uses a cooling-liquid-path provided to a frame. Other cooling systems have also been used, e.g., cooling the core coils directly in a motor or generator with oil or dissipating the heat inside a motor through a heat-pipe to the outside. However, since these methods cool the inside of motor or generator directly, the number of components inevitably increases, which makes the motor or generator structure complicated, and thus, produces another problem, such as, maintaining the reliability of the motor or generator.
The coil of the stator assembly which is a heat source is electrically insulated on its surface so that the coil can carry electric current. An insulator or an insulating paper is disposed between the coil and an iron core of the stator assembly that is made of electromagnetic steel sheets in order to prevent the coil from being peeled off its sheath or broken by the edges of the iron core when the wires are coiled. The insulating paper generally is an aramid paper. The above discussion describes in general the structures of the motor-coil.
These insulators and insulating papers are electrical insulating materials and at the same time, they are heat insulators and thus block heat conduction. For instance, the aramid paper's heat conductivity is as low as 0.14 W/mK.
A highly heat-conductive resin can be disposed between the coil and the iron core of the stator assembly to efficiently dissipate the heat from the motor. This arrangement increases heat-dissipation-efficiency, but it also increases the motor's weight and becomes a critical problem particularly in a motor for an electric vehicle. Such a motor needs to be downsized and a greater output at higher efficiency is demanded. Further, this arrangement requires equipment and a process for potting the heat conductive resin. Reliability of the motor must be maintained to avoid an electrical breakdown of coils due to shorting out of wires and depends on the pressure and/or temperature used during resin potting.
If the material of an insulator is changed to a higher heat-conductive material, the following problem occurs: In a process of winding a wire on a core, the wire requires some tension, otherwise, the wire becomes loose, and the wire can not be wound correctly within a slot of the core. The insulator material needs to have at least a sufficient level of strength to withstand this tension. On the other hand, an electrical insulator of high heat conductivity, such as, silicone rubber or synthetic resin containing aluminum oxide of excellent heat-conductivity is well known but is soft and fragile and thus has poor strength and is not useful as an insulator for the coil. Also, if the motor is exposed to thermal cycles and thermal shock, the interface between the coil insulator and core can delaminate due to the difference of coefficient of linear thermal expansion. If a de-lamination at the interface exists, heat transfer is drastically restricted by the presence of air which has low thermal conductivity at 0.02 W/mK.
To address this problem, plastic compositions having improved thermally conductive properties have been developed. Neal, U.S. Pat. No. 6,362,554 discloses a method of encapsulating a high speed spindle motor that includes a core and a stator having multiple conductors. These conductors create magnetic fields as they conduct electrical current. A thermally-conductive body encapsulates the stator. The '554 Patent discloses that a thermally-conductive, but non-electrically-conductive, plastic composition including filler particles can be used to form the encapsulating body. According to the '554 Patent, a preferred form of plastic is polyphenyl sulfide, and the amount and type of filler can be a ceramic material, glass, Kevlar® aramid fiber from E. I. DuPont de Nemours and Company, carbon fibers or other fibers.
U.S. Pat. No. 6,600,633 (Macpherson, et al.) discloses a thermally conductive over-mold for a disc drive actuator assembly but does not recognize the advantage of coating a stator core with an adhesive component.
U.S. Pat. No. 6,509,665 (Nishiyama, et al.) discloses a motor having stator with insulator of high heat conductivity but there is no indication of the advantage of coating a stator core with an adhesive component.
U.S. Pat. No. 7,077,990 (Miller) discloses high density, thermally conductive plastic compositions for encapsulating motors but there is no indication of the advantage of coating a stator core with an adhesive component.
Although the use of such thermally-conductive plastic compositions can be somewhat effective in transferring heat away from the stator assembly compared to using a general plastic compositions, there is a need of further improvements to provide heat transfer between the stator core and the over-molded plastic. An adhesive component intervening between the stator core and the over-mold of a thermally conductive resin can improve heat transfer between them that leads to efficient heat release from the stator assembly.
The present invention provides such a stator assembly over-molded with a thermally conductive polymer composition that has an adhesive component at the interface between the polymer composition and the stator core to improve heat transfer.